Pulse combustion drying, employing a pulse combustor essentially similar in nature to a pulse jet engine, is a relatively recent but recognized technique for effecting the drying of particulate materials. Illustrative of some earlier endeavors in pulse jet field for drying and other purposes are U.S. Pat. Nos. 3,618,655; 4,226,668; 4,226,670; 4,265,617 and 2,838,869. In the first of these patents a plurality of pulse jet engines are mounted at the base of a vertical chamber. A paste or slurry of the particulate material to be dried is introduced into the exhaust duct of such pulse jet engines which function to at least partially dry the particulate material and introduce it into the chamber where induced vortex gas flow causes circulation of the particulate material and consequent opportunity for further drying thereof. In the latter of these patents a linear pulse jet engine assembly for projecting various types of materials is disclosed.
A current state of the art pulse combustion dryer is made and sold by Sonodyne Industries of Portland, Oregon. The pulse combustor unit, which is the heart of the drying system, is a specially contoured and generally U-shaped hollow tube whose dimensions and materials of construction determine its operation. The pulse combustion process is initiated when air and fuel from a constant low pressure supply thereof are drawn into the combustion chamber portion of the combustor and ignited by a spark. Hot gases created by the resulting detonation move in both directions from the combustion chamber. In one direction, they pass through an inlet nozzle and adjacent air augmenter, and in the other direction, through U-shaped exhaust section and past a raw material injection port at the downstream end thereof. Detonation in the combustion chamber causes the pressure therein to rise, momentarily shutting off the fuel supply. As the combustion chamber pressure falls following detonation, fuel is again admitted and mixed with air being drawn through the inlet nozzle. Detonation occurs again, either because of contact between the explosive air-fuel mixture and the spark or by contact with the sufficiently hot wall of the chamber itself. Once the wall temperature reaches approximately 1800.degree. F., the spark can be extinguished and the process becomes self-igniting.
The pressure fluctuation, which causes the pulsing behavior of the combustor, results in strong standing waves of sound energy which move in both directions from the chamber. Repeated detonations also create high speed displacement of hot gases with about 90% thereof exiting through the tailpipe and associated exhaust system components. Introduction of moisture laden particulate material into the downstream end of the exhaust sections subjects such material to the sound waves which, although not fully understood, are believed to break the bonds between the solid particulate matter and the liquid, most often water, and in an atomization of the water into fine droplets with a consequent increase in surface area for evaporation. The heat present in the exhaust gas interacts with the atomized cloud of introduced raw material allowing highly efficient evaporation to occur. During drying, the rapid evaporation of the water absorbs most of the heat and the solid particulates are maintained and exit in a relatively cool state. It should be noted that while operating temperatures in the pulse combustion exhaust system exceed 2500.degree. F., the residence time of the raw product solids in contact with the exhaust gases is very short, being in the order of a few milliseconds. Because of such short residence time and the high heat consumption effected by evaporation, the temperatures of the dried solid particulates rarely exceeds 100.degree. to 150.degree. F.
While pulse combustion drying apparatus of the type described immediately above has proved to be both efficient and economical in the drying of many diverse materials, the basic U-shaped configuration of the combustion chamber and downstream exhaust portions thereof has created certain assembly and maintenance problems as well as introducing certain system instabilities.